Optical devices are widely used in communications applications because optical fibers can carry significantly more information than conventional copper wires can carry in the same space. Optical devices are also being used to connect computer components such as circuit boards and even individual circuits because of their great capacity and relative immunity to interference.
The small size of optical components, while allowing a great many components to be assembled in a small volume, makes the components difficult to assemble and connect to each other. Optical fibers, for example, are typically only a few thousandths of an inch in diameter. They are composed of a core, a cladding, and a coating. The cladding has an index of refraction slightly less than that of the core, and the coating has an index of refraction that is slightly greater than that of the cladding. The core, cladding, and coating are typically made of silica glass. Light propagates in the core and is reflected off the cladding, thereby guiding the light within the core. The coating protects the fiber from the environment and provides mechanical strength to reduce unacceptably tight bending of the fiber.
Light can propagate in an optical fiber in any number of vibrational modes. Fibers that support a single mode are called “single mode fibers” and fibers that support multiple modes are called “multi-mode fibers.” The width of a typical single-mode optical fiber core is about 9 microns (0.0004 in.). The cladding diameter is typically about 125 microns (0.005 in). The diameter of a multi-mode fiber core is typically about 50 μm (0.002 in) and the diameter of the cladding is typically about 125 microns (0.005 in). The index of refraction of the core of some multimode fibers, known as graded index fibers, changes gradually as the distance from the axis increases.
Because of the small diameter of the optical fibers, it is difficult and expensive to accurately align the fibers with each other and with other components. For example, when two optical fibers are coupled, the axes of the fibers need to be aligned within a few microns so that the light exiting the core of one fiber enters the core of the next fiber. A misalignment between the cores of a single micron will result in greater than a ten percent loss of light. Because only light that enters the fiber within an “acceptance angle” will propagate down the fiber, the angles between the fibers that are coupled must be close to zero. Similarly, light exits the fiber in the shape of a cone. Connecting fibers must be positioned as close to each other as possible so that most of the cone of line light exiting one fiber is within the core of the next fiber.
Moreover, to reduce loss of light as it leaves an optical fiber, the optical fiber must be properly “terminated” before connecting to other fibers or other components. The optical fibers must be cut at a precise angle and its end polished. The process of preparing the fiber end, or terminating the fiber and aligning it is time consuming and therefore expensive.
Different optical components perform different functions with regard to light signals so it is necessary to route light using optical fibers and waveguides to different components to perform different operations. For example, light may be generated by a laser, routed through an optical fiber, switched to an alternate fiber, multiplexed into a fiber with light of a different wavelength, separated out for the different wavelengths, and then detected by a detector. Typically, light is routed between components by optical fibers or other waveguides.
Optical components can be classified as either active or passive. Passive devices include, for example, optical fibers that carry light and couplers that route light between fibers. Active devices include, for example, light emitting diodes and lasers that generate light and detectors that detect light and convert the light signal into an electrical signal. One type of laser that has become widely used in optoelectronics is the vertical cavity surface emitting laser, or VCSEL. A VCSEL is typically fabricated on a semiconductor die or chip and emits light perpendicular to the surface of the chip.
Optical components, including optical fibers, are frequency grouped to process multiple signals in parallel. Such groups include arrays of lasers or photodiode detectors and cables containing multiple optical fibers. Optical components perform functions such as detection, optical signal branching, optical multiplexing, optical switching, and optical modulation.
Optical components such as laser and photodetectors are typically assembled into packages. Aligning the components within the package and aligning the fiber with the package are time consuming, expensive operations. Moreover, the optical fibers that are routed to the packages must be aligned with the components in the package.
The cost of aligning and connecting the components within the package is one of the major costs of producing the optical packages and of using the optical package. When connecting optical fibers, it is typically necessary to “actively align” the fibers to the devices, that is, light is transmitted though the device and the connection is monitored during the alignment process. For example, when aligning optical fibers to a laser, the laser will be operating and a detector is connected to the opposite end of the fibers. The fiber is adjusted in relation to the laser until the light detected at the other end of the fiber is maximized. Once alignment is obtained, the position of the components relative to each other is fixed, typically by an ultraviolet curable adhesive or solder, and the assembly is typically either hermetically sealed or surrounded with a plastic material. The fiber is typically metal coated and soldered to seal with the package and to fix it in place.
To complicate the alignment problem, many components require electrical connections, as well as optical fiber connections. For example, lasers need an electrical power supply and information signals are provided to modulate the laser beam. As another example, photodetectors provide electrical signals that correspond to the intensity of the received light. Electrical signals are conducted to and from components by fine wires that are bonded to conductive pads on the components. The resulting assembly is fragile and is often sealed to protect it from the environment.
Many connection schemes have been proposed to facilitate the connection between different types of optical components. For example, “V”-shaped grooves etched into a component package can be used to align optical fibers with the component. Many of the mechanical connectors that align optical components are etched from silicon or machined from ceramic to meet the tight tolerances.
U.S. Pat. No. 6,222,967 to Adano et al. for “Packaging Platform, Optical Module Using The Platform, and Methods for Producing the Platform and Module” describes a system that uses a package having grooves to position optical fibers and having horizontal and vertical reference surfaces to position components in three dimensions in relation to the fibers. The fiber and components are fixed in position by a cover plate or by an adhesive. U.S. Pat. No. 6,233,383 to Artigue et al. describes another system using reference surfaces for aligning optical components.
U.S. Pat. No. 6,227,724 describes a method for constructing an electro-optical assembly that uses a flexure element that can be deformed to bring an optical component into alignment with other components on a substrate. The flexure element is then fixed to the substrate.
U.S. Pat. No 5,574,814 to Noddings et al. describes a method of assembling an optical transceiver with an optical connection. A laser is used to precision drill slots for the connector tooling pins in the connector body, so that the fibers in the connectors align accurately with the transceiver. This technique enables a passive assembly technique as opposed to active alignment using optical power coupling to align the device.